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LISA DESJARDINS, CNN CORRESPONDENT (via telephone): You'll see at some of the visitor entrances especially. They do have technology where they will swipe -- they'll use a piece of cloth and they will swipe a bag or a purse and then run a quick test on the residue. So that happens, but that is not done for every person entering the complex. That's done, as in some airports, just randomly. And apparently there was no reason for this man to be stopped according to those regular security procedures at the Capitol.

What seemed to really raise the alarm bells here is that he was able to deliver letters in person against protocol that somehow members of some Senate offices accepted packages and letters in person that they should not have. And that word apparently got to Capitol Police in time for them to find this man here in the Capitol complex, in the Hart Building I'm told, and then stop him and they are now questioning him.

WOLF BLITZER, CNN ANCHOR: And I assume they'll be going through those suspicious envelopes and the backpack as well.

All right, standby, Lisa Desjardins. She's working this story. She's in the Hart Senate Office Building, which is being evacuated right now.

Dana Bash is standing by. She's over at the Russell Center Office Building. Brianna Keilar is over at the White House. We're awaiting a briefing there.

Here's what we know at the top of the hour. Let's just reset for our viewers here in the United States and around the world.

We're watching some potentially significant developments in Washington. Yesterday we learned that a suspicious envelope did reach the office of a United States senator, Senator Wicker of Mississippi. An envelope that was tested not once, not twice but three times and so far proven to be positive for ricin. But authorities say they need more testing. They're sending that letter to Fort Detrick, Maryland, where the U.S. government does these kinds of tests for suspicious letters. A lot of nerves frayed as a result of this letter.

And now a second letter they have stopped. A suspicious letter similar to the letter sent to Senator Roger Wicker of Mississippi, addressed to the president of the United States. That letter is now being investigated as well.

At the same time, we've heard of these evacuations and the freezes, if you will, from various Senate office buildings. All of this happening in the aftermath of the Boston Marathon bombings. So nerves are frayed right now. Everyone is on edge as the president of the United States gets ready to come to Boston tomorrow to participate in an interfaith memorial service.

So let's go to the White House. Brianna Keilar is standing by.

Brianna, I assume the president, he's all set. He's coming to Boston tomorrow for this memorial service.

BRIANNA KEILAR, CNN WHITE HOUSE CORRESPONDENT: Yes, that is the expectation, Wolf. There have been no changes to his schedule. He is here at the White House today where he's had some meetings and been briefed on the latest in the Boston bombings, where he's really trying to carry on with at least some of the plans that he had in place for this week, sort of signaling that what happened in Boston on Monday isn't going to stop him certainly and shouldn't stop Americans from proceeding with their lives.

He'll be hosting some bicyclists who are part of the Wounded Warrior Project Soldier Ride. Today he'll be meeting with Democratic senators offsite at a restaurant in D.C., as he's met so often with lawmakers recently.

But the headline coming out of the White House right now, Wolf, is that a letter addressed to President Obama that contained a suspicious substance, and that's all that we're being told, was intercepted at an offsite mail processing facility. This is a mail facility run by the Secret Service where they get all the mail addressed to the White House and they check it for this very reason, to screen it, to do scientific tests, to make sure that it's safe to come here.

It had a suspicious substance in it. We are told, Wolf, by a law enforcement source, that there are similarities to the letter sent to senator Wicker. What exactly does that mean, I've asked my sources. So far, Wolf, inconclusive at this point. We do know there were some marks, certainly some, I guess you could say identifying marks in the Wicker letter. A postmark from Memphis, Tennessee. There was no return sender. We don't know if that or -- nor the message that was included in the letter to Senator Wicker, if that bears the similarity. But we are being told that there are similarities. So it certainly makes you wonder if it could have been sent from the same person or if they think -- if authorities think it is. And we're certainly digging on that at this point.

Meantime, Wolf, you can see in the lower corner of the screen, we're waiting right now the White House briefing. It was scheduled for 11:45 a.m. I will tell you, it's not that unusual for it to run a little later than what it is now. So we do know, of course, the White House press secretary, Jay Carney, will be receiving a lot of questions about these developments. But at this point it's also very much an issue, Wolf, for the FBI as they look into exactly, you know, what has gone on with these letters and who may be responsible for it, Wolf.

BLITZER: And if -- or there is -- maybe there is, maybe there isn't, some connection to the Boston Marathon bombings. It may be totally unrelated. It may be some copycat individual maybe wanting to playoff these Boston Marathon bombings. We have no idea if there's any connection, if this is all simply coincidental. But it is pretty scary when you think about it.

I take it, Jessica, we're waiting for Jay Carney. This is his regularly scheduled daily briefing. It was supposed to start, what, about 15, 20 minutes ago. So it's running a little late. As Brianna says that's not unusual at all. But the circumstances of today's briefing are dramatic.

BLITZER: Jessica, I think your audio -- we're going to have to fix that audio. Standby for a moment, Jessica.

Let's bring in Dana Bash up on Capitol Hill right now just to reset what's going on. The very latest.

Dana, tell our viewers in the United States and around the world what we know and maybe even, more importantly, what we don't know.

DANA BASH, CNN CHIEF CONGRESSIONAL CORRESPONDENT: Well, let me start with giving you something new in the last few minutes. I got an e-mail from the communications director for Senator Richard Shelby, whose office is the one that is kind of in lockdown, down the hall from where I am here on the third floor of the Russell Office Building. And this e-mail from Jonathan Bofao (ph) says that "the Capitol Police are investigating a suspicious package delivered to our front office." He has no more information about this.

But this is completely in keeping with what our Lisa Desjardins reported a while ago that what Capitol Police are doing is they are talking to, questioning a suspect who came into the Hart Office Building and potentially the same person came to this building to deliver, against protocol, to deliver a letter to the front office. And when I say it's against protocol, it's because, as we have been talking about with regard to these letters sent with what may be the poison ricin, that's all done at an offsite facility for this very reason. Mail is not supposed to come directly to the Capitol because they want it to be screened now offsite.

So that's where things stand here in the Russell Building. We are still now not allowed to go down the hall toward Senator Shelby's office, but obviously this building is still not evacuated because we're here.

Same goes for the Hart Building there. The people who are in the offices are still being told to stay in their offices. So it's not being evacuated. But there's kind of a modified lockdown there.

Again, the other thing I can tell you is that from our Joe Johns, who is talking to law enforcement officials, they say that they do not believe that what's happening now with these lockdowns and the suspicious packages here as we speak, doesn't appear to be related to the letters sent to Senator Roger Wicker and to President Obama which with what appears to be the poison ricin. But I want to emphasize that what I just said, we do not -- they do not believe because, as you've been saying, Wolf, and it's important for us to underscore, everything is fast moving. We are getting information, as law enforcement officials are, and they're trying to figure out what the genesis of this is, what the source is and I don't think anybody really has a true idea of what's related to other incidents and what isn't.

BLITZER: Yes, it may certainly be coincidental. But I have to tell you that the timing of all of this raises so many fears, so many suspicions. Only a couple days after the terrorist bombings here at the Boston Marathon and now these suspicious letters may be laced with this poison ricin. May be laced with this poison ricin reminds me, I assume a lot of our viewers out there, of the anthrax-laced letters that occurred shortly after 9/11.

It may simply be coincidental. We don't know. There's so much we don't know. We're waiting for this White House briefing. Jay Carney, the White House press secretary, supposed to have started this briefing about 20 minutes or so ago. But it's been delayed for clearly understandable reasons.

I think we've reconnected with Jessica Yellin, who's in the Briefing Room right now.

What are they saying, Jessica, about the delay of Jay Carney's briefing?

YELLIN: Wolf, if I can just start with some news. I've just received a statement from the FBI saying that the second letter that was sent to the offsite White House facility addressed to the president did test positive for ricin on a preliminary test. The envelope addressed to, again, the White House, was quarantined and is part of now a coordinated investigation headed up by the FBI. This statement notes that operations here at the White House were in no way impacted. Again, it's nowhere -- it never reached any area that touches the president in any way.

This statement also adds that filters at a second government mail screening facility - filters -- also tested positive for ricin this morning on a preliminary basis. Now, I've been previously told that preliminary tests, as we know, sometimes test positive and then you find out a negative test later. So that's why they're being cautious. So they say that second facility is now -- they're testing the mail at that second facility now.

They say any time any suspicious powder is found in a mail facility, field tests are conducted and you get inconsistent results. They're conducting full analyses. I'm sort of reading this as I just received it. And those tests can take 24 to 48 hours. So the investigation is ongoing.

And they end, of course, talk about burying the lead, quote, "there is no indication of a connection to the attack in Boston." So statement from the FBI, which again heading up the investigation there on the ground also with the people on the ground in Boston, saying there is no indication of a connection to the attack in Boston. These letters found in Washington so far not linked to that event, Wolf.

BLITZER: Yes, there was apparently no indication either. We still don't know with the anthrax-laced letters back in the aftermath of 9/11 in 2001 if there was any connection to 9/11. But a lot of this raising suspicions. A lot of questions being raised. And we're going to try to come up with answers. I know law enforcement is doing their best to come up with answers as well. And we do expect, Jessica, the White House press secretary to show up in that Briefing Room where you are fairly soon. Are they saying that briefing is still scheduled?

YELLIN: Yes. We just got our two-minute warning. Just so nobody reads too much into the delay in the briefing, sometimes these briefings are delayed and run late a half hour, even 45 minutes for no big announcement at all. It just is sort of the course of events here. So we got a two-minute warning. That actually can mean anywhere from two to 20 minutes. So we'll be here ready for you when he comes out.

BLITZER: So that would be a loose two-minute warning as we used to say.

YELLIN: Very loose indeed.

BLITZER: A very fluid situation over there.

All right, standby for a moment and we'll see if Jay Carney walks out within the next minute or so.

Tom Fuentes, our law enforcement analyst, is standing by as well. He's a former assistant director of the FBI.

A quick question before we go to Jay Carney, Tom. If the tests - if these tests, the preliminary tests, one, two, three preliminary tests show a positive reading for ricin, a poison, a deadly poison, are those usually accurate or they -- or when they send them to Fort Detrick in Maryland to more sophisticated labs, they prove out to be inaccurate?

TOM FUENTES, FORMER FBI ASST. DIRECTOR: No, they do prove out to be inaccurate. There's a very high number of false positives with the tests on ricin poison. And the test yesterday, the very first letter that arrived yesterday, there were multiple tests that differed with each other in the field. So you had the first test that was positive, second test negative, third test inconclusive. So really the FBI is relying on the definitive testing that will come from Fort Detrick and the scientists there conducting a very extensive and professional test of the substance in that letter. And again, that will continue on with the letter --

BLITZER: And that lab in Fort Detrick -- yes, that lab in Fort Detrick, that's where they sent all the anthrax-laced letters as well. That's the most sophisticated lab that the U.S. government operates when it comes to these kinds of poisons, is that right?

FUENTES: That's correct. They can deal with poisons like this. They can examine weaponized anthrax, for example, and determine what strain, what particular strain of it, it may be. And that's why really they're waiting for that examination.

These preliminary tests are just that, they're preliminary. If something suspicious, get it out of there and do the real exam. And in the cases right now, because of the sensitivity of these things occurring right after the Boston attack, that's why there's extra sensitivity and multiple field tests conducted. But, really, they're relying on the text from Fort Detrick.

It will be the same thing with the letter or letters that were addressed to the White House. And has been mentioned, all letters to these government agencies go to an offsite. None of them -- anybody out there that's mailing a letter to whatever senator or congressman or the White House, the president, it's not going to go directly there. It's going to go through this screening process. Unless you have an individual bypass the process, as Dana is talking about at the office building right now, the individual being questioned having a backpack of letters, that is suspicious.

But on the other hand, this could be somebody that's advocating some cause and just wants to directly make sure that he gets the letters to the appropriate offices of, you know, members of The Hill to - as again, you know, kind of a, you know, a lobbyist on the cheap, so to speak. So we just don't know that yet. That will require the additional questioning. And actually to see the letters that he's got. What is he delivering? What does it say? Is it just about some cause or what? So that will -- we'll find the answers to that, I'm sure, very quickly.

BLITZER: I'm sure we will because if they have someone that they're questioning right now and they have the backpack, they have the suspicious letters, they're going to go through that.

Tom, stand by for a moment. Once again we're awaiting the press secretary at the White House, Jay Carney, he's supposed to go into that briefing room fairly soon and brief all of us on what we know, what we don't know, a suspicious letter addressed to the president of the United States, a very similar letter that at least in one of the preliminary tests proved to be positive for ricin sent to Republican Senator Roger Wicker of Mississippi that was discovered earlier this week.

And now at an offsite, a letter, similar letter, addressed to the president of the United States. We're anxious to see if there's any connection. We don't know if there is any connection to the aftermath of the Boston marathon bombing.

I'm here in Boston reporting on what's going on. Joe Johns is our crime and justice correspondent. What are you picking up, Joe? JOE JOHNS, CNN CRIME AND JUSTICE CORRESPONDENT: Well, the first thing, Wolf, and I think it's a fairly important point to make that it's inconclusive so far.

We're not certain that this is ricin, but if it is, it's important not to say that no one was in danger because it didn't reach the White House ...

JAY CARNEY, WHITE HOUSE PRESS SECRETARY: I know there are several matters that I'm sure you'll be interested in discussing today, a couple of issues that are under investigation by the FBI, and I thought I would start with that and then take your questions. As you saw from a statement from the FBI. as well as a statement from the United States Secret Service, there was a letter sent to, addressed to the president that-at an off-site mail facility. It was noticed to have contained a suspicious substance, and tests were undertaken. The FBI has the lead in that investigation, of course, and has said in its statement that they will be conducting further tests to determine what the nature of the substance is. Of course, there was another letter, as you know, sent-or detected by Capitol police that was sent to a United States senator. That also has been-is the subject of the investigation by the FBI. And for more information about these matters, I would refer you to the FBI. The president-I'm sure you'll ask this-the president has, of course, been briefed on these letters. He was briefed last night and again this morning. Secondly, there is obviously a lot of interest in the explosions in Boston, and I wanted to make clear, as you heard from the president yesterday, that our hearts and prayers go out to the victims and to their families who were injured, those who were injured, as well as killed in this heinous and cowardly act. The full weight of the federal government is behind this investigation, which is being led by the FBI. And as the president said, we will find out who did this, we will find out why, and we will bring those responsible to justice. That said, it is very important that we allow this investigation to run its full course and to ensure that we retain the integrity of that investigation. Therefore, on matters related to the investigation, I would direct you to the FBI. As you know, the FBI is giving the press briefings on the ground in Boston, and I believe they will be holding another briefing today. The president, as you know, has been briefed regularly on the incident in Boston, beginning almost immediately after it took place. This morning, the president again convened a meeting in the Oval Office with his national security team on the ongoing investigation. Participating in that briefing was the attorney general, Eric Holder, the FBI director, Robert Mueller, the president's chief of staff, Denis McDonough, his national security adviser, Tom Donilon, his homeland security adviser and counterterrorism adviser, Lisa Monaco, Deputy National Security Adviser Tony Blinken, White House counsel Kathy Ruemmler, deputy chief of staff Alyssa Mastromonaco, Deputy National Security Adviser Ben Rhodes, and the vice president's national security adviser, Jake Sullivan. As you know, tomorrow morning, the president will travel to Boston to speak at an interfaith service dedicated to those who were gravely wounded or killed in Monday's bombing. I have a scheduling update for you on that. The first lady will be joining the president on the trip to Boston. With that, I will take your questions.

QUESTION: Thank you, Jay. Obviously, the public is already

pretty nervous, given the situation in Boston. Has the FBI told the White House anything about whether the letters that have been received are related to the Boston incident?

CARNEY: I would point you to the statement that the FBI put out. It is my understanding that they have not made that connection, but I would refer you to the FBI for that.

QUESTION: Do you know if they're not making that connection or if they've ruled out the connection?

CARNEY: I would refer you to the FBI.

QUESTION: So at this point, there's nothing you can say to the public to reassure them...

CARNEY: Well, I can tell you that in line-as you know, for a long time now, there have been long-established procedures and protocols. Any time a suspicious powder is located in a mail facility, it is tested. And I would underscore that the mail is screened-the mail sent here is screened and that these tests are undertaken at remote sites to mitigate the risk both to those recipients and to the general population.

The FBI has the lead for determining whether a suspicious powder is a dangerous substance, such as ricin, and those take place at accredited facilities, and they take a certain amount of time, as the FBI has indicated.

You know, these procedures are in place. The procedures are effective and in operation now. And we are in the midst of that process which the FBI is undertaking at this time.

QUESTION: But, again, there's nothing you can say about whether there's any connection between the letters and the Boston incident?

CARNEY: I would point you to the FBI, which has the lead investigation-the lead in the investigation into each matter and what they have said about this and-and their assessments on that- on any connection between these two matters.

QUESTION: I imagine you'll probably point me to the FBI on this, as well, but I'll ask. Is there any indication on-on the Boston explosions on whether this looks to be a foreign terrorist incident or a domestic incident?

CARNEY: What the president said yesterday remains true today, which is that there is an active investigation ongoing. We have the full weight of the federal government behind this investigation being led by the FBI. All components of the federal government are assisting, including the intelligence community and others, assisting state and local authorities and the FBI in this investigation.

As the president said, we do not know at this time yet whether it was an organization or an individual, foreign or domestic. But we will find out, and we will hold accountable and bring to justice whoever is responsible. But this investigation is now not even 48 hours old, and it is important that we maintain the integrity of the investigation. It is important, as both state and local law enforcement officials and government officials, as well as federal officials, have made clear that the American people provide whatever information they might have that could be of assistance in this investigation.

There is an 800 number, 1-800-CALL-FBI, which has been put out to provide a method for individuals who might have information to contact law enforcement on this matter. And that is absolutely the responsible thing to do.

But the details of the progress of the investigation, assessments about-preliminary assessments about who may be responsible I will not make. And I think that, as a matter of upholding the integrity of the investigation, others will not, as well. Yes?

QUESTION: Jay, there are still police tape up across or around Pennsylvania Avenue in front of the White House. Is that a temporary measure? Or is-has there been any other threats to the White House that-other than these letters?

CARNEY: I'm not aware of any. I think the Secret Service said yesterday that out of-you know, through normal procedures and through an abundance of caution, that certain actions are taken, but I would refer you to them for any details about the perimeter and any- any actions the Secret Service might be taking today.

QUESTION: Is the president discouraged at all by the fact that there aren't more leads in the Boston bombings?

CARNEY: I wouldn't say one way or the other, because I think it's important to allow the investigators to do their work. He is being briefed on this regularly. He had a substantial meeting earlier today. Lisa Monaco, his counterterrorism and homeland security adviser, is updating him continuously throughout the day and the evening and the night on these matters.

But I wouldn't characterize his view of the investigation, because that would then characterize the investigation.

QUESTION: What will his message be in Boston tomorrow?

CARNEY: It will be one of resolve. It will be one of the commonality that we all feel as Americans with the people of Boston and those who are visiting Boston for the marathon and who both endured this horrific event and then demonstrated their bravery in its immediate aftermath.

I mean, I would obviously ask you to wait until you hear his remarks, but as you heard him say yesterday from this podium, the way that the people of Boston and the city of Boston responded reminds us and reminds the world of just who we are as a people.

QUESTION: And the last question. How did he react to the briefings about the letters that were addressed to him?

CARNEY: The president was briefed on these matters. I don't have a way to characterize his reaction. Obviously he understands and we all understand that there are procedures in place as the FBI has said, there are-there is is a process in place that ensures that materials that are suspicious or substances that are found to be suspicious at remote locations are then sent for secondary and more intense testing, and that process is under way now.

QUESTION: Jay, following on Julie's question, at the end of the FBI's statement they say there is no indication of a connection to the attack in Boston. Why-they won't explicitly rule out a connection nor will you. Why is that?

CARNEY: Well I would point to the FBI which you have just cited where they say there is no indication. That's a pretty clear statement.

QUESTION: But there's still the possibility of a connection, so it would be reassuring to stand there and say there is no connection. What's the significance?

CARNEY: I think it is up to the investigating authorities to say. They have made a fairly clear statement which I was suggesting to Julie they -- that-that was available and the statement they put out.

I am not going to assess it further from here. I would remind you that these investigations both of the bombings in Boston and the -- the letters in question here are just under way, and, you know, I wouldn't presume to characterize the nature of the investigations. I would point to the statement by the FBI as you said that they have made that says they have no indication of a connection between the two.

QUESTION: There are a lot of betters now. Look, first, may I clarify? The statement refers to two different facilities. We also know of a letter that was sent to Capitol Hill. Can you clarify, is that three facilities and letters total or just two?

CARNEY: I would-I would refer you to the FBI. My understanding of -- what I am aware of is a letter sent to a member of the Senate and a letter sent to the president. But it-but I- there have been reports I think the FBI has said of other potential letters. I don't know if that's the case. I would refer you to the statement by FBI.

QUESTION: And given the number of letters and possible packages

suspicious packages, is this concerning to the administration there is something else going on here, that there is somebody trying a follow on, a lone wolf, somebody trying to capitalize on the concern after Boston?

CARNEY: I think those are questions that investigators are looking to answer. I would point you to what the FBI has said already, what the Secret Service has said and make clear that we have had procedures in place for quite some time now when it comes to the processing of mail sent to the White House complex that ensure that, that mail is processed at an off site facility to mitigate any risk to potential recipients here and to the general public, and that those procedures have been in place for some time and have been effective.

I would ask you to check with the FBI to see what assessments they will make along the lines you're asking and this is again very early.

QUESTION: Can I ask one more which is is this something that happens every so often and we just don't hear about it because there is not as much interest in this kind of thing or is this an unusual occurrence?

CARNEY: Well, again, the FBI and the Secret Service are best to ask these questions of, because they handle the processing of-or the investigation in one case and the Capitol police and the Secret Service handle the assessments of it.

It is certainly the case I think as the FBI pointed out that when preliminary tests are done, secondary and I suppose tertiary tests are made because of the need for accuracy and to be sure about what the substances might might be. But for the history of this, I would refer you to-to the FBI and the Secret Service and the Capitol Police.